1. Field of the Invention
This invention relates to a method of foaming and curing silicone gum stocks by using microwave.
2. Description of the Prior Art
It is known that silicone rubber can be foamed or sponged by using various blowing agents which release nitrogen or carbon dioxide gas. Berridge in U.S. Patent No. 2,857,343 describes using blowing agents such as dinitrosopentamethylenetetramine and N,N' -dimethyl-N,N'-dinitrosoterephthalamide with a combination of two peroxide catalysts, tertiary butyl perbenzoate and either benzoyl peroxide or bis-(2,4-dichlorobenzoyl)peroxide to make silicone rubber sponge. Berridge in U.S Pat. No. 2,875,163 describes using a heating process before adding the catalysts and blowing agent to improve the final product. Wade and Blanchard in U.S. Pat. No. 2,951,819 describe making cellular rubber-like articles using stearic acid and alkyl metal borohydrides as the blowing agent. Wade et al. also describe using sodium carbonate and p,p-oxybis(benzene sulfonyl hydrazide) as blowing agents for natural and synthetic rubber-like materials. Smith in U.S. Pat. No. 3,238,157 describe making cellular silicone rubber by mixing silicone gums with particulate, solid, void-producing matter, by curing the mixture to a silicone elastomer and thereafter leaching out the particulate matter thereby forming the cellular silicone rubber.
Bond and Tomita in U.S. Pat. No. 3,271,332 describe making a room temperature vulcanizable siloxane foam by adding a basic vulcanization catalyst, such as dibutyltindilaurate to a mixture of 60 to 80 parts by weight of a diorganopolysiloxane fluid gum free of hydroxyl termination having a viscosity of 1,000 to 50,000 cps. at 25.degree. C., 40 to 20 parts by weight of a benzenesoluble silicone resin composed of R.sub.3 SiO.sub.0.5 and SiO.sub.2 units, sufficient methyl hydrogen polysiloxane to provide from 0.017 to 0.17 part by weight silanic hydrogen and between 0.1 to 2 parts by weight of a basic vulcanization catalyst active to split off silanic hydrogen.
Modic in U.S. Pat. No. 3,425,967 describes a foamable mixture comprising 100 parts of a vinyl chain-stopped polysiloxane, 10 to 100 parts organopolysiloxane copolymer of R.sub.3 SiO.sub.0.5 and SiO.sub.2 units having 2.5 to 10 mole percent silicon-bonded vinyl groups, 10 to 100 parts asbestos or fibrous potassium titanate, 0 to 50 parts of a finely divided inorganic filler, a platinum catalyst, an amount of liquid organohydrogenpolysiloxane to provide from 0.5 to 1.0 silicon-bonded hydrogen atoms per silicon-bonded vinyl group and 1 to 5 parts of a blowing agent, such as azo-isobutyronitrile, dinitrosopentamethylenetetramine, benzenesulfonhydrazide, N,N'-dinitroso-N,N'-dimethylterephthalamide, p,p'-oxy-bis-(benzenesulfonhydrazide), terephthalazide and azodicarbonamide.
Wada, Itoh and Kuga in U.S. Pat. No. 3,677,981 describe foamable silicone elastomeric compositions comprising 80 to 99 parts diorganopolysiloxane gum having a viscosity of at least 1,000,000 cs. at 25.degree. C. and 0.025 to 0.25 moles of vinyl group, 1 to 20 parts of diorganopolysiloxane having a viscosity of at least 10 cs. at 25.degree. C. and at least ten times as many moles of vinyl group as the gum, an amount of organohydrogenpolysiloxane such that the Si-H linkages are from 50 to 200 mole percent of the total vinyl groups and there are at least three Si-H per molecule, 20 to 100 parts of silica, a catalytic amount of a platinum catalyst and 1 to 10 parts blowing agent such as azobisisobutyronitrile, dinitrosopentamethylenetetramine, N,N' -dimethyldinitrosoterephthalamide and diaminobenzene.
Other than Bond et al. cited above, these references describe heat activated and curing foams. Siloxane foams have also been prepared at room temperature by combining hydroxylated organopolysiloxanes, silicon-bonded hydrogen and tin salts of carboxylic acid such as described by Bruner in U.S. Pat. No. 3,070,555, by Nitzsche et al. in U.S. Pat. No. 3,338,847, Hersh in U.S. Pat. No. 3,429,838, Joyce in U.S. Pat. No. 2,956,032 and Nitzsche et al. in U.S. Pat. No. 3,428,580. More recently, Schuyler B. Smith in an application Ser. No. 519,380, filed Oct. 30, 1974, now U.S. Pat. No. 3,923,705 and assigned to the same assignee as this application described a method of making organosiloxane foams by mixing an organohydrogensiloxane having at least three silicon-bonded hydrogen atoms per molecule, a hydroxylated organosiloxane having from greater than 1.0 to 2.5 silicon-bonded hydroxyl radicals per molecule and a platinum catalyst wherein the molar ratio of silicon-bonded hydrogen atoms to silicon-bonded hydroxyl radicals is from 2.5 to 40 and the viscosity of the mixture is less than 100,000 cps. at 25.degree. C. Smith also describes a method wherein there is also present a benzene-soluble vinyl containing triorganosiloxy endblocked polydiorganosiloxane.
Foams or sponges described above use external blowing agents or use tin salts with silicon-bonded hydrogen to make the foam. The external blowing agents are useful but many are dangerous to handle or there is difficulty in regulating the gas formation from the blowing agent and the curing at the same time. The prior art patents have therefore made various contributions to overcome this problem. Additionally the resulting foam properties required improvement and composition variations were also described by these references to improve the foam properties. Foams could be prepared using tin salts but these foams were not as useful as hoped for, because of properties and stability over broad temperature ranges. Smith in Ser. No. 519,380 describes a more suitable foam which could be used in areas where fire retardancy was required, however, Smith's method required use within a short period of time, once the ingredients were all mixed. It was therefore unexpected that a platinum catalyst could be used to make gum based stocks foam because they require substantial mixing, such as on mills, to prepare the compositions and premature foaming and curing would certainly not be desirable, especially if it occured on the mill.
Where these spongeable compositions are described in an application Ser. No. 595,918, filed on even date herewith July 14, 1975, by CHi-Long Lee and Gary M. Ronk entitled "Spongeable Silicone Gum Stocks" which is assigned to the same party as the present application. This application discloses compositions comprising 100 parts by weight of a polydiorganosiloxane gum base stock, 15 to 60 parts by weight of an organosiloxane having silicon-bonded hydroxyl radicals, 3 to 25 parts by weight of an organohydrogensiloxane, 0.05 to 0.5 parts by weight acetylenic alcohol inhibitor and 1 to 200 p.p.m platinum as a platinum catalyst. Foamable gum base stocks are also described in an application Ser. No. 595,920, filed July 14, 1975 by Ronald E. Kittle entitled "Foamable Silicone Gum Stocks Having Less Skin Thickness" assigned to the same party as the present application. The compositions described by Kittle are similar to those described by Lee and Ronk except the 15 to 60 parts by weight of organosiloxane having silicon-bonded hydroxyl radical is replaced by 5 to 30 parts by weight of an organic alcohol having one C--OH per molecule.